Mars Section Circular, 2001 December

2001 Opposition

General

The previous Section Circulars were issued during the 1999 apparition. Just this one Circular is being issued for 2001, because Mars has not been well-placed for observation from the UK for the majority of 2001. It is understandable that many found that the planet was simply too low to observe properly at opposition from the UK, which was a great pity due to the large disk diameter at opposition, and to the exceptional planet-encircling dust storm which developed on June 26 and lasted until last month. At the moment Mars is relatively high and free from dust, yet almost nothing is being done in the UK apart from my own work. Back to your ‘scopes, please!

Observers

The following contributed work in 2001, the majority from outside the UK: M. Adachi, G. Adamoli, D. Bates, S. Beaumont, N. Biver, S. Buda, T. Cave, A. Cidadao, E. Colombo, E. Crandall, B. Curcic, P. Devadas, T. Dobbins, K. De Groff, M. Di Sciullo, S. Ebisawa, C. Ebdon, N. Falsarella, M. Frassati, M. Gaskell, E. Grafton, W. Haas, T. Haymes, A. Heath, M. Hendrie, C. Hernandez, T. Ikemura, M. Justice, T. Leong, F. Melillo, C. Meredith, M. Minami, D. Moore, D. Niechoy, B. Pace, P. Parish, D. Parker, T. Parker, D. Peach, C. Proctor, T. Richards, R. Schmude, W. Sheehan, I. Stellas, G. Teichert, D. Troiani, A. Valimberti, A. Gonzalo Vargas, and the writer. Thanks to all.

Review of the 2000–01 Session

This was published by the Director in the BAA Journal for 2001 October. As noted therein, there was Mars news in BAA Circulars 777 and 779. It is assumed that all UK members subscribe to the BAA Circulars. This is where brief news of the great dust storm of 2001 was published. There was also a good deal about how to observe, and an early Interim Report, in the Journal for 2001 June.

The Great Dust Storm of 2001

Origins

Before opposition, MGS imaged local dust storms, and one example in Hellas is illustrated in Astronomy and Geophysics, 2001 August, p.26. The image is dated April 8. There is further MGS evidence of local dust activity in Hellas in mid-June. By late June, ground-based observers also detected local activity in Hellas. On the 24th, atmospheric dust was confined within the N. part of the basin, but on the 26th a long, bright, twisting ribbon of dust had extended into Ausonia, marking the emergence of an important event at Ls = 185 deg., right at the start of southern spring. The HST imaged Mars the same day, but this activity was beyond the evening limb. Only the slightly dusty nature of Hellas could be seen on the latter image. But MGS temperature data (using the thermal emission spectrometer) show a warming beginning in Hellas on the 24th. The event developed quickly, and dust expanded from Hesperia (long. 270 deg.) and Hellas (long. 300 deg.). Leong’s image of the 27th shows additional dust in Libya with diffuse dust starting to mask the Syrtis Major and points east. Rapid expansion of the event occurred chiefly to the E. and N.E., and additional activity occurred over Elysium. Within a few days the OAA were calling it a ‘global’ storm, but this is to misuse the agreed classification system. The dust covered much of one hemisphere, true, but in longitudinal extent it was still only a large regional event. Over the next few days Syrtis Major was effaced. So far, this was not much different from the course of many past regional storms such as that of 1988 June.

The Director published several letters in the Communications in Mars Observations of the Oriental Astronomical Association (OAA – Japan) No.247, 2001 July 10. These provide some historical perspective.

Development

On the night of July 3/4 a significant development began: a new bright dust core appeared in Daedalia in the images of Valimberti. This new storm expanded rapidly, and its incidence showed that a global forcing condition was operating despite the very early seasonal date. Over the next few days Parker and others imaged the new event’s expansion primarily to the east over Solis Lacus, Valles Marineris and Mare Erythraeum (etc). Many small new bright clouds appeared around the region. The storm front crossed Noachis to link with the Hellas regional event, and the latter event had by then expanded east to meet the new event around the longitude of Thaumasia/ Mare Sirenum. By July 11 (storm day 16) the planet was encircled by dust and contrast was low everywhere. Albedo features rapidly faded from view. A pair of HST images for June 26 and September 4 published in Sky and Telescope, 2002 January, give a graphic representation of the extent of the dust.

By mid-July the colour of the planet was more yellow than orange and even to the naked eye the colour was noticeably different. The dust veil extended down to a latitude of about 40 deg. north, so that the north polar hood was not veiled. The NPH became less active in August, though this could be due to the southward movement of the subsolar point. The dust had a significant warming effect upon the martian atmosphere, to the extent of 40 K or more between latitude 20 N up to the S. pole, as measured by MGS from orbit. All evidence of white cloud activity was suspended for months. Indeed, even the limb brightening was less sharp and less marked during the storm.

Viewed from the Earth it was hard to see what was happening in the far south. The SPC was tilted away from the Earth and the cap, though large, was foreshortened. It appears that dust did not cover the SPC (see the aforementioned HST image for September 4) but nevertheless did extend to rather high southern latitudes.

During July the only specific bright clouds were over Hellas and also over Daedalia. Several observers reported renewed activity over Daedalia, and this source remained active for some time. In August and September many observers sketched or imaged Olympus Mons as a dusky spot, showing that the storm could not have been much higher than its summit caldera. As Minami points out, the spot represents the caldera making a hole in the surrounding swirling yellow clouds rather than an albedo feature as such. Sure enough, as the dust settled, the caldera became indistinguishable.

Clearance

These notes are intended as preliminary only, and do not constitute a full report. From late August onwards a gradual clearing was underway, but it was very slow, and throughout September the ground markings remained hard to see well. By October the general albedo features could be easily recognised, but contrast was not back to normal until mid or late November. Signs of atmospheric dust were even then still detectable: some dust hung over Edom crater and a patch of dust hung over Argyre, whilst Hellas was still bright and yellow. In early December McKim and others still saw N. Hellas to be bright in yellow and red light, but there were no other bright dust clouds. By then all the features were sharp and well-defined, even if some were apparently not as dark as in the pre-storm period, and furthermore the SPC was again well contrasted as the southward tilt of the axis increased, though by then greatly shrunken. The storm had lasted a long time, but not quite as long as the truly global event of 1971.

Historical context

For a NASA press conference on October 11, at which HST and MGS data upon the great storm were to be presented, Jim Bell asked for the writer’s views about the ‘2001A’ storm. Here is what I e-mailed in reply:

‘I think the most important point about the present storm (2001A) is that it was seasonally the earliest ever recorded amongst all the past encircling events. It was also one of the most enduring storms, and optically one of the most dense... It also may mark the return to the dusty climatic period that was witnessed throughout nearly the whole of the 1970s and into the early 1980s. My historical work clearly established the fact that encircling storms were witnessed every martian year from 1971 to 1977. It showed that 1975 contained a planet encircling storm – a fact not widely appreciated – and this year together with 1971 and 1973, when coupled with the Viking data up to late 1977 show the emergence of a great storm every year on Mars (with two in 1977). This epoch was unprecedented, and Viking’s cameras recorded an atypical Mars: a fact well worth reiterating. During this epoch both Hellas and Thaumasia/Daedalia dominated the scene as emergence sites; indeed they remain the only such sites for the emergence of encircling storms.

‘The 2001A event is important in possibly marking a return to dusty climatic conditions: but by definition we shall not know for at least two or three more martian years.

‘The development of the 2001 event was similar to many past great storms... It began with what appeared to be a large regional event originating in the Hellas longitude. In this respect the storm was not seasonally especially early. Historically, regional storms from Hellas had begun at even lower Ls (see the tables in the BAA Memoir (Mem. Brit. Astron. Assoc., 44 (1999)). One more discovery from my book. I divided up the last century and a half into epochs: 1866–97; 1898–1929; 1930–61; 1962–93. I wrote on p.144 in connection with the Hellas emergence site: ‘In the last three epochs, the R(egional) type storms have begun progressively earlier in the season.’ It seems that the 2001A event’s timing has also been unusually early. Is there a simple physical mechanism for this phenomenon, associated with the gradual net accumulation of dust grains at a site?

‘Event 2001A became planet-encircling when a secondary source of dust emergence commenced in Daedalia about July 4, and together with the initial dust outbreak carried dust around the globe. All the great storms have become encircling in this way when a secondary source supported the initial outbreak elsewhere: for example, the 1973 event began in Daedalia/Claritas, but it had secondary outbreaks over Hellas and Meridiani Sinus.

‘There is a possibility that the retreat of the SPC began early in 2001: I have made no measures yet, and it is just a possibility. In 1986, the SP hood cleared at Ls 180, but in 2001 the hood was already cleared by then.

‘The 2001A storm showed a more vigorous expansion to the north than some of the most recent encircling events. Thus the area of Syrtis Major was blotted out from view quickly. As to its duration, it is still in evidence, though clearly dying out now. Thus its duration will be greater than 15 weeks, or 105 days. It will probably last no more than another few weeks. The durations of the planet-encircling storms of the 1970s–80s dusty epoch were (my data): 1971, 161 days; 1973, 91 days; 1975, 100 days; 1977A, 60 days; 1977B, 158 days; 1982, 110 days.

‘For visual observers 2001A was probably the most opaque storm since 1971, although the 1971 storm was more global in its dust coverage. Of course, the 1977 and 1982 Viking data were a little incomplete.’

The MGS data for the 2001A event have been written up in the paper ‘Thermal Emission Spectrometer Observations of Martian Planet-Encircling Dust Storm 2001A’, by M.D. Smith, B.J. Conrath, J.C. Pearl and P.R. Christensen, and it was recently accepted for publication in the journal Icarus.

Changes upon the martian surface

As is usual with big dust storms, some changes upon the martian surface were apparent. Prior to the storm, the markings were much as they had been in 1999. Nepenthes was invisible, Cerberus and Trivium Charontis nearly so; Nodus Alcyonius, the Amenthes darkening and Solis Lacus were all prominent. After the storm, Parker remarked that Syrtis Major appeared thinner, and the writer found it more tapering to the north in his recent work; at the same time, Deltoton Sinus had become more visible. And there was a new dark marking just W. of Solis Lacus, greatly resembling the old ‘Phasis’ canal shown on the maps of G.V. Schiaparelli and N.E. Green from 1877. This was imaged from early September onwards, when Solis Lacus itself was still mostly hidden by bright dust clouds. (This feature was also observed during the mid-1980s through the early 1990s.) Solis Lacus itself was smaller after the storm, and the feature known as Nectar, connecting Solis Lacus to Mare Erythraeum, was greatly faded.

The rest of the apparition

Mars is still very much observable, upon the meridian early in the evening, though he is now only some 7 arcsec across. Work should continue into 2002 as far as possible until Mars is lost in the evening twilight. There may even be further dust activity, given that we are still within the martian southern hemisphere summer season, and accurate recording of the size and shape of the SPC is important for comparison with the historical records. The Director is enjoying good views at present, despite the tiny diameter. The markings are clearly defined upon the strongly phased disk, and the planet is crowned by a small SPC summer remnant.

Richard McKim, Director